#include<bits/stdc++.h>
using namespace std;

double D[30][30],P[30],C[30][30];
int N,K;
int main(){
	ifstream cin("dezastru.in");
	ofstream cout("dezastru.out");
	cin >> N >> K;
	for(int i = 1;i<=N;i++) cin >> P[i];
	for(int i = 0;i<=N;i++) D[i][0] = 1,C[i][0] = 1;
	for(int i = 1;i<=N;i++)
		for(int j = 1;j<=K;j++) D[i][j] = D[i-1][j] + D[i-1][j-1]*P[i],C[i][j] = C[i-1][j] + C[i-1][j-1];
	
	cout << D[N][K]/C[N][K];
	return 0;
}